1. Field of the Invention
This invention relates to a process for growing single crystals of alpha-aluminum orthophosphate and alpha-gallium orthophosphate.
2. Description of the Prior Art
Alpha-aluminum orthophosphate (berlinite) and alpha-gallium orthophosphate (GaPO.sub.4) are among several alpha-quartz isomorphs that have for decades been synthesized for research purposes. An attempt to grow large single crystals of berlinite began after World War II, in an effort to find new piezoelectric crystals for frequency control applications. The project ended a few years later, because success was achieved in quartz crystal growth and because quartz was considered superior for the piezoelectric devices known then. Specifically, it was concluded that berlinite had a lower Q and lower coupling coefficient than quartz. Furthermore, tests on both X and Y cuts to berlinite plates, showing a negative frequency drift with increasing temperature, indicated that there was little chance of finding a zero temperature cut similar to the AT cut of quartz.
Interest in berlinite was renewed in 1976, when Barsch and Chang found that berlinite does have temperature-compensated cuts and that the coupling coefficient for surface acoustic wave (SAW) devices can be four times greater than for quartz.
Several processes for preparing berlinite have been reported in the technical (W. Jahn et al., Chem. Erde 16, 75 (1953); J. M. Stanley, Ind. Eng. Chem. 46, 1684 (1954); E. D. Kolb et al., J. Crystal Growth 43, 313 (1978)); and L. E. Drafall et al., RADC-TR-80-73, Final Technical Report, March, 1980.
In a typical process, seed crystals are suspended near the bottom of a vertical autoclave and nutrient powder is suspended in a basket at the top. Both seed and nutrient are immersed in concentrated phosphoric acid. In one variation of the process, the temperature is raised slowly from about 150.degree. C. over a period of days. In another variation, a temperature gradient is maintained in the autoclave. In some cases, a gradient is maintained as the temperature is slowly raised.
Although most of the processes reported involve growth in phosphoric acid, two recent reports on growth in hydrochloric acid have appeared. Detaint et al., Proc. 34th Symp. on Freq. Control, p. 93 (1980), grew berlinite crystals from powder by increasing the temperature 4.degree. C. per day during hydrothermal growth in HCl. Kolb et al., J. Crystal Growth 51, 178 (1981), achieved higher growth rates in HCl at comparable conditions to those used for H.sub.3 PO.sub.4 growth. In their method, a temperature gradient of about 1.degree.-3.degree. C. was maintained as the temperature was increased from about 150.degree. C. to about 200.degree. C. or higher at 2.degree.-20.degree. C./day.
None of the prior art methods for growing berlinite consistently provide crystals having the high degree of quality--i.e., freedom from defects--needed for SAW device applications.